This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing a primer bridge ignited by a high voltage pulse.
High voltage flashlamps may be divided historically into three categories: (1) those having a spark gap within the lamp such that electrical breakdown of a gaseous dielectric (e.g., the combustion-supporting oxygen atmosphere) is an integral part of the lamp ignition mechanism; (2) those having a conductive primer bridge that electrically completes the circuit between the lead-in wires; such primers are rendered conductive by additives such as acetylene black, lead dioxide, or other electrical conduction-promoting agents; and (3) lamps having an essentially nonconducting primer bridge that connects the inner ends of the lead-in wires and which becomes conductive, upon application of a high voltage pulse, by means of breakdown of the dielectric binder separating conductive particles therein.
The earliest high voltage flashlamps were of the spark gap type construction wherein an electrical spark would pass through the gaseous atmosphere within the lamp. The spark would jump between two electrodes, at least one of which was coated with a primer composition. Such lamps tend to exhibit poor sensitivity and reliability when flashed from low power sources such as the miniaturized piezoelectric devices that are suited for incorporating into pocket-sized cameras. Most of the electrical input energy in such lamps is lost to the gas atmosphere by the spark. Also, the electrical characteristics vary considerably from one lamp to another because of shreds of metallic combustible in the spark gap and consequent variations in effective gap length.
The use of spaced lead-in wires interconnected by a quantity of electrically conductive primer gives rise to highly predictable behavior and a well-defined electrical path through the lamp. Here again, however, relatively highpowered flash sources must be used in order to attain reliable lamp flashing.
Present state of the art flashlamps of the high voltage type make use of a bridge of initially nonconducting primer to interconnect the inner ends of the lead-in wires. Considerably higher sensitivity is attainable by this method, apparently because the breakdown and discharge follow a discrete path through the primer composition and thereby promote greater localized heating. With respect to specific construction, such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass reservoir of some type. A mass of primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at an initial fill pressure of several atmospheres.
Lamp functioning is initiated by application of a high voltage pulse (e.g., several hundred to several thousand volts, as for example, from a piezoelectric crystal) across the lamp lead-in wires. The mass of primer within the lamp then breaks down electrically and ignites; its deflagration, in turn, ignites the shredded combustible which burns actinically.
The fabrication and testing of a number of different ignition structures has shown several problem areas that are peculiar to high voltage type flashlamps, and which are familiar to those skilled in the art of flashlamp design. For example, random location of the shreds of metallic combustible can cause short circuiting of the lead-in wires or interfere with the intended electrical breakdown path through the primer. Post-flash short circuiting can be caused by primer residue, metallic or semimetallic droplets of slag from the ignited shreds of combustible, and possible welding of the lead-in wires after ignition.
An example of a prior art lamp structure directed to overcoming some of those problems is described in U.S. Pat. No. 3,875,260 of Cote wherein one of the lead-in wires of the ignition mount is recessed in a glass insulating sleeve which is sealed to the press at one end and open at the other end. The other lead-in wire is formed so that it rests against and terminates slightly above the open end of the sleeve. The mass of primer material is disposed to cover the open end of the sleeve and bridge the ends of the lead-in wires. The glass sleeve has a side vent opening for the purpose of avoiding air entrapment during primer application to assure the primer material reaches the sleeved lead. Such a vent hole, however, introduces a degree of added cost and exposes the sleeved lead-in wire to a possible shred shorting condition. Consequently, an alternative approach that has been employed is to use a continuous sleeve, with no vent hole. But this last-mentioned mount design also has some apparent shortcomings. The fact that the sleeved lead-in wire is recessed causes problems with primer bridging. It is necessary to use air pressure to force primer into the glass sleeve to contact the lead. This method consists of a seal connecting the top edge of the primered bottles and using the same seal as a means to force primer into the sleeve. Poor sealing of the bottle caused by a slight chip in the glass, worn or torn sealing edge, etc., can cause splashed primer and primer not contacting the lead in the sleeve. Another criticism of the prior construction is the possibility of shreds getting into the sleeve opening. Since the primer is being forced into the sleeve, an opening can appear in the primer, enhancing the possibilities of shred shorts.
Another prior art lamp structure of interest is described in U.S. Pat. No. 3,884,615 of Sobieski wherein the two lead-in wires of the ignition mount are sealed into a doughnut-shaped glass bead which is open at both ends. The central opening in the bead is filled with a mass of primer material which bridges the lead-in wires. This construction uses the bead as a shield to keep the combustible fill away from the bare lead wire below the bead. The bead obviously must be smaller than the inside diameter of the lamp envelope. However, this creates a space for strands of fill to slip past the bead and come in contact with the lead wires, thereby shorting out the system and rendering the lamp inoperable. The close proximity of the bead and lamp envelope requires precise mount placement in order to prevent the bead from being sealed into the lamp envelope, thus weakening the final product. The Sobieski patent does disclose alternatives to counter the shred short problem, such as the use of a sleeve below the bead or special bead shaping, but such designs add to the cost of a bead structure, which is in itself comparatively expensive, and introduce additional manufacturing problems.